Improve binary expressions to preserve whitespace at operands

[mgreter]

@xzyfer couldn't resist to fix this 🙈 I hope you can incorporate this easily with your parsing refactor. This here doesn't touch the static value parsing and should be pretty well self contained.

This should currently pass this specs (sass/sass-spec#624):

div {
  baz: #{1/2}/3;
  baz: #{1/  2}/  3;
  baz: #{1  /2}  /3;
  baz: #{1  /  2}  /  3;
}

add {
  baz: #{1+2}+3;
  baz: #{1+  2}+  3;
  baz: #{1  +2}  +3;
  baz: #{1  +  2}  +  3;
}

sub {
  baz: #{1-2}-3;
  baz: #{1-  2}-  3;
  baz: #{1  -2}  -3;
  baz: #{1  -  2}  -  3;
}

mul {
  baz: #{1*2}*3;
  baz: #{1*  2}*  3;
  baz: #{1  *2}  *3;
  baz: #{1  *  2}  *  3;
}

mod {
  baz: #{1%2}%3;
  baz: #{1%  2}%  3;
  baz: #{1  %2}  %3;
  baz: #{1  %  2}  %  3;
}

Fixes #1722

[xzyfer]

Spec activated sass/sass-spec#657